Problems of fouling are caused by the attachment and growth of juvenile mollusks in service and cooling water systems, and the settlement of young adults in condenser tubes of condenser water systems, causing deleterious effects to the operation and safety of these systems. In fossil-fueled systems, problems have been related to plugging of condenser tubes, surface water heat exchangers, and fire protection systems. In nuclear power plants, additional problems of blockage may occur, including the shutdown of service water and emergency reactor cooling systems.
Among the most serious threats posed by C. fluminea is its macrofouling of nuclear and fossil-fueled power generating stations. In power plants, the shells of living and dead clams foul steam condensers and service water systems. Clams enter these systems as juveniles or adults carried on water currents and settle, grow, reproduce and accumulate in numbers that reduce water flow to levels that seriously compromise or prevent operation. (Goss et al., Control studies on Corbicula for steam electric qeneratinq plants, J. C. Britton (Ed.), Proceedings, First International Corbicula Symposium, Texas Christian University Research Foundation, Fort Worth, Tex., pp. 139-151 (1977)).
C. fluminea is a particularly dangerous macrofouling species in nuclear power plants because it simultaneously fouls primary and secondary (backup) systems, thus compromising fail-safe operation Henagar et al., Bivalve Fouling of Nuclear Power Plant Service-Water Systems. Factors That May Intensify the Safety Consequences of Biofouling, NRC FIN B2463, NUREG/CR-4070, PNL-5300, Vol. 3 Div. Radiation Programs and Earth Sciences, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, D.C., 51 pp. (1985)). Major biofouling incidents have been reported at nuclear power stations in Arkansas (Arkansas Nuclear I), Brown's Ferry, Alabama, and Baldwin, Ill. (Henegar et al., above). Such incidents have led to the issuance of a bulletin by the U.S. Nuclear Regulatory Agency (United States Nuclear Regulatory Agency (USNRC), Flow Blockaqe of Cooling Water to System by Corbicula sp. (Asiatic Clam) and Mytilus sp. (Mussel), Bulletin No. 81-03, Office of Inspection and Enforcement, United States Nuclear Regulatory Commission, Washington, D.C. 6 pp. (1981)) requiring all nuclear power stations in the U.S. to inspect for and report the presence of this species in their operations and raw water sources. Analysis of this and other data has indicated that of the 32 nuclear power stations within the known geographic range of C. fluminea in the U.S., 19 already report infestations of varying severity and 11 others are in close proximity to known populations (Counts, Distribution of Corbicula fluminea at Nuclear Facilities, NRC FIN B8675, NUREG/CR-4233, Div. Engineering, Office of Nuclear Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington, D.C. 79 pp. (1985)). Thus, macrofouling by C. fluminea presently poses a dangerous and costly problem in the nuclear industry.
Within the known geographic range of C. fluminea in the United States lie hundreds of fossil-fueled electric power stations whose raw water systems are also subject to macrofouling by this species. As in nuclear plants, such macrofouling requires expensive shut-downs for repair and replacement of damaged equipment, as well as expensive and often futile retrofitting of anti-fouling equipment that has generally proved ineffective in controlling clam impingement.
While a number of control methodologies have been developed to reduce the macrofouling of industrial and power station service water systems by C. fluminea, none has proved completely effective.
Control of C. fluminea macrofouling in power station and industrial service and auxilliary water systems has primarily been through chlorination. Recommended residuals of chlorine are 0.5-1.0 .mu.g per liter for continuous application or 500 .mu.g per liter for periods of 100-500 hrs. to kill juvenile clams borne on intake currents into these systems (Cherry et al., Corbicula fouling and control measures at the Celco Plant, Virginia, Am. Malacol. Bull. Special Ed. No. 2, pp. 69-81 (1986); Mattice, Freshwater macrofoulinq and control with emphasis on Corbicula, Symposium on Condenser Macrofouling Control Technologies: The State of the Art, Electric Power Research Institute, Palo Alto, Calif., pp. 4-1-4-30 (1983); Sinclair et al., Further Studies on the Introduced Asiatic Clam (Corbicula) in Tennessee, Tennessee Stream Pollution Control Board, Tennessee Department of Public Health, Nashville, 76 pp. (1963)).
As chlorination is generally only allowed by U.S. Environmental Protection Agency regulations for 2 of every 24 hrs. in systems returning service water to source (United States Environmental Protection Agency (USEPA), Effluent limitations guidelines, pretreatment standards and new source performance standards under Clean Water Act; steam electric power generating point source category, 40 CFR, Parts 125 and 423, Fed. Regist. 45(200):68328-68337 (1980)), it has proved to be generally ineffective in controlling C. fluminea macrofouling (Page et al., Biofoulinq of power plant service water systems by Corbicula, Am. Malacol. Bull. Special Edition No. 2: 41-45 (1986)). Heavier chlorination may also exacerbate corrosion of pipes, and when C. fluminea burrows into accumulations of corrosion products and silt in the low flow areas of these systems it effectively becomes insulated from the toxic effects of chlorination (Johnson et al., Engineering factors influencing Corbicula fouling in nuclear service water systems, Am. Malacol. Bull. Special Ed. No. 2:47-52 (1986)).
A number of molluscicides other than chlorine have been tested for efficacy in control of C. fluminea, but have proved ineffective or impractical (Mattice, above). Antifouling paints, coverings and slow release toxic pellets appear effective in killing clams (Mattice, above), but their relatively short half-lives, and difficulties in application, make their utilization in existing service water systems neither feasible nor cost effective.
Therefore, there is a major incentive for the development of an environmentally safe, cost effective, highly potent molluscicide to control macrofouling by C. fluminea in industrial and power generation raw water systems. To date no such molluscicide has proven to be completely satisfactory for the control of C. fluminea macrofouling in the raw water systems of power stations or other industrial operations.
The biology of bivalve mollusks, including such species as C. fluminea (Asiatic clam), is especially suited for their establishment and growth in the water systems of power plants. The Asiatic clam occurs in great abundance in fresh water systems. McMahon and Williams (McMahon et al., A reassessment of growth rate, life span, life cycles and population dynamics in a natural population and field caged individuals of Corbicula fluminea (Muller) (Bivalvia: Corbiculacea), Am. Malacol. Bull. Special Ed. No. 2, pp. 151-166((1986)) measured a population of 1000 clams per square meter in the Trinity River and Benbrook Lake area in Texas. Since power generating stations require a large quantity of service water, they are located on major streams or lakes. The water is drawn from the supply source through a canal. Clams find these canals to be favorable for the production of their larval offspring which may be many thousands per clam. The larval stages and small adults are small enough to pass through the screens used to retard the passage of detritus into the plant. The larvae will then attach themselves to surfaces by their suctorial foot and the elaboration of mucilaginous byssal attachment threads.
Once attached, the juveniles mature into adults. In one to three months the juveniles and small adults can grow in size so that when carried by currents into the condenser tubes, they can lodge in the tubes and cause the accumulation of small particles of material behind them, thereby completely plugging the tube. If enough tubes become plugged in this manner, the flow of water through the condenser is reduced to levels which seriously affect its efficiency, thereby forcing unit shut-down and manual removal of accumulated shells and other debris.
Clams do not grow in the condenser tubes, but are carried there by the currents from the water supply, particularly the embayment following screening. Juvenile clams carried into service water systems will mature in situ, and such systems will be plugged both by the adults produced in place and by those which are brought in by currents. Therefore, the control of fouling may be accomplished by killing the adult clams, the juvenile clams, or by preventing the attachment of the juveniles to surfaces.